The present invention relates to an objective lens driving apparatus for an optical pickup for use in an optical disc player such as a compact disc player (CDP) and a video disc player (VDP), and more particularly to an objective lens driving apparatus for improving the structure of a tracking coil wherein current for controlling the tracking of optical recording media is supplied to control the tracking accurately.
Generally, players using optical recording media such as a CDP and a VDP, include an optical pickup for injecting light beams and detecting the light beams reflected from the information recorded surface of an optical disc. The optical pickup comprises an objective lens, a lens holder, tracking coils, a focusing coil, a permanent magnet and a light source. The objective lens is held by the lens holder, the focusing coil is wound around the vertical outer periphery of the lens holder, and the tracking coils are fixed on the focusing coil. The permanent magnet is located outside the focusing coil and the tracking coils, and is opposed to the above two coils. The light source is installed on the lower part of the lens holder, and the beam irradiated from the light source is projected to the information recorded surface of the optical disc via the objective lens.
In such an optical pickup, a current signal obtained from a detecting beam reflected from the information-recorded surface of the optical disc during recording/reproducing, is supplied into the focusing coil and the tracking coils, so that a servo control for correcting the location of the objective lens by the electromagnetic circuit which is formed by the coils and a permanent magnet is performed. However, such an optical pickup traces a fine error signal to correct the light beam to be precisely projected on the accurate information track of the optical disc, so that a high accuracy is required. Accordingly, when correcting the location of the track of the objective lens, it becomes necessary to prevent the lens holder from rolling due to an electromagnetic force unnecessary for controlling the tracking, due to the structure of the tracking coils.
FIG. 1 is a partially cut-away perspective view showing a conventional objective lens driving apparatus for the optical pickup.
Referring to FIG. 1, reference numeral 1 designates an objective lens opposed to an optical disc (not shown). Objective lens 1 is held by a lens holder 2. Lens holder 2 is finely movably supported by four supporting rods 4 attached to a fixed member 3. Each supporting rod 4 is formed of elastic material such as rubber. A focusing coil 5 is wound around a vertical outer periphery of lens holder 2, and tracking coils 6, in a rectangular shape, are fixed to an outer periphery of focusing coil 5. A pair of permanent magnets 8 retained by yokes 7 are located outside focusing coil 5 and tracking coils 6 to oppose the above coils.
In the conventional objective lens driving apparatus for the optical pickup as mentioned above, lens holder 2 is driven by an electromagnetic circuit made by the current flowing in focusing coil 5 and the magnetic force of permanent magnets 8, so that the focusing correction in the direction of an optical axis of objective lens 1 (Y-axis), is performed. Further, lens holder 2 is driven by an electromagnetic circuit made by the current flowing in both sides of tracking coils 6 and the magnetic force of permanent magnets 8, so that lens holder 2 is positionally corrected in the direction along the information recorded surface of the optical disc (X-axis). In such a tracking correction operation, an electromagnetic force unnecessary for controlling the track of lens holder 2 is generated in the upper and lower parts of each tracking coil 6. That is, if the direction of a magnetic field (direction B) and the direction of a current (represented by dotted lines) flowing in tracking coils 6 are the same as that in FIG. 1, electromagnetic forces F1 and F2 are generated in the upper parts of each tracking coil 6 and electromagnetic forces F3 and F4 are generated in the lower parts of each tracking coil 6. Electromagnetic forces F5 versus F6 and F5 versus F7 having opposite directions are generated along the X-axis on both sides of each tracking coil 6. Electromagnetic forces F1 and F2 and F3 & F4 generate opposite turning forces in the upper and lower parts of each tracking coil 6.
However, as shown in FIG. 2, when lens holder 2 moves in the Y-axis by a focusing correction signal, the magnetic flux density of the upper parts of each tracking coil 6 differs from that of the lower parts. Accordingly, the electromagnetic forces generated in the upper and lower parts of each tracking coil 6, causes a difference in the turning forces such that lens holder 2 is inclined. As a result, since lens holder 2 of the optical pickup, which demands high accuracy in correcting the location, loses balance thus the light beam is emitted while being inclined with respect to the disc surface, so that accuracy is lowered. Also, an electromagnetic force F5 in the inner parts and electromagnetic forces F6 and F7 in the outer parts, both generated by the current flowing in the vertical portions of each tracking coil 6, are generated in opposite directions. Therefore, lens holder 2 is driven in the X-axis by the difference, so that the driving region of lens holder 2 is narrowed.